Generation of Magnetic Field by Combined Action of Turbulence and Shear

Yousef, T. A.; Heinemann, T.; Schekochihin, A. A.; Kleeorin, N.; Rogachevskii, I.; Iskakov, Alexey B.; Cowley, S. C.; McWilliams, James C.

doi:10.1103/physrevlett.100.184501

Cited by 118 publications

(175 citation statements)

References 28 publications

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“…Concurrent nonlinear direct numerical simulations of unstratified shear dynamos in Cartesian boxes [12,13] have confirmed all results discussed in Sec. IV for the low-Rm regime [12,54]. First, we see a qualitative change in the kinematic dynamo with the addition of rotation, due to the change in sign of the η yx transport coefficient [12].…”

Section: Discussionmentioning

confidence: 98%

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Squire¹,

Bhattacharjee²

2015

Phys. Rev. E

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This article presents a calculation of the mean electromotive force arising from general small-scale magnetohydrodynamical turbulence, within the framework of the second-order correlation approximation. With the goal of improving understanding of the accretion disk dynamo, effects arising through small-scale magnetic fluctuations, velocity gradients, density and turbulence stratification, and rotation, are included. The primary result, which supplements numerical findings, is that an off-diagonal turbulent resistivity due to magnetic fluctuations can produce large-scale dynamo action-the magnetic analog of the "shear-current" effect. In addition, consideration of α effects in the stratified regions of disks gives the puzzling result that there is no strong prediction for a sign of α, since the effects due to kinetic and magnetic fluctuations, as well as those due to shear and rotation, are each of opposing signs and tend to cancel each other.

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Section: Discussionmentioning

confidence: 98%

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Squire¹,

Bhattacharjee²

2015

Phys. Rev. E

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“…Recent work has explored the possiblity that non-helical turbulence in conjunction with background shear may give rise to large-scale dynamo action [5][6][7][8][9][10]. The evidence for this comes mainly from direct numerical simulations [5][6][7], but it by no means clear what physics drives such a dynamo. One possibility that has received some attention is the shear-current effect [10], where an extra component of the mean electromotive force (EMF) is thought to result in the generation of the cross-shear component of the mean magnetic field from the component parallel to the shear flow.…”

Section: Introductionmentioning

confidence: 99%

Transport coefficients for the shear dynamo problem at small Reynolds numbers

Singh

Sridhar

2011

Phys. Rev. E

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We build on the formulation developed in Sridhar & Singh (JFM, 664, 265, 2010), and present a theory of the shear dynamo problem for small magnetic and fluid Reynolds numbers, but for arbitrary values of the shear parameter. Specializing to the case of a mean magnetic field that is slowly varying in time, explicit expressions for the transport coefficients, α il and η iml , are derived.We prove that, when the velocity field is non helical, the transport coefficient α il vanishes. We then consider forced, stochastic dynamics for the incompressible velocity field at low Reynolds number.An exact, explicit solution for the velocity field is derived, and the velocity spectrum tensor is calculated in terms of the Galilean-invariant forcing statistics. We consider forcing statistics that is non helical, isotropic and delta-correlated-in-time, and specialize to the case when the meanfield is a function only of the spatial coordinate X 3 and time τ ; this reduction is necessary for comparison with the numerical experiments of Brandenburg, Rädler, Rheinhardt & Käpylä (ApJ, 676, 740, 2008). Explicit expressions are derived for all four components of the magnetic diffusivity tensor, η ij (τ ) . These are used to prove that the shear-current effect cannot be responsible for dynamo action at small Re and Rm, but for all values of the shear parameter.

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“…Moffatt 1978;Parker 1979;Krause & Rädler 1980;Pouquet et al 1976;Blackman & Brandenburg 2002). Alternatively, there is emerging agreement that a combination of shear and fluctuating kinetic helicity can conspire to produce an LSD (Vishniac & Brandenburg 1997;Brandenburg 1995;Brandenburg 2005;Yousef et al 2008;Heinemann et al 2011;Mitra & Brandenburg 2012;Sridhar & Singh 2013). The EMF can also be sustained by magnetic helicity fluxes, either local (within sectors of a closed volume) or global (fluxes thought the system boundary) as we will later discuss.…”

Section: Types Of Dynamos and Approaches To Study Themmentioning

confidence: 99%

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

Blackman¹

2016

Space Sciences Series of ISSI

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Magnetic fields of laboratory, planetary, stellar, and galactic plasmas commonly exhibit significant order on large temporal or spatial scales compared to the otherwise random motions within the hosting system. Such ordered fields can be measured in the case of planets, stars, and galaxies, or inferred indirectly by the action of their dynamical influence, such as jets. Whether large scale fields are amplified in situ or a remnant from previous stages of an object's history is often debated for objects without a definitive magnetic activity cycle. Magnetic helicity, a measure of twist and linkage of magnetic field lines, is a unifying tool for understanding large scale field evolution for both mechanisms of origin. Its importance stems from its two basic properties: (1) magnetic helicity is typically better conserved than magnetic energy; and (2) the magnetic energy associated with a fixed amount of magnetic helicity is minimized when the system relaxes this helical structure to the largest scale available. Here I discuss how magnetic helicity has come to help us understand the saturation of and sustenance of large scale dynamos, the need for either local or global helicity fluxes to avoid dynamo quenching, and the associated observational consequences. I also discuss how magnetic helicity acts as a hindrance to turbulent diffusion of large scale fields, and thus a helper for fossil remnant large scale field origin models in some contexts. I briefly discuss the connection between large scale fields and accretion disk theory as well. The goal here is to provide a conceptual primer to help the reader efficiently penetrate the literature.

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Generation of Magnetic Field by Combined Action of Turbulence and Shear

Cited by 118 publications

References 28 publications

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Transport coefficients for the shear dynamo problem at small Reynolds numbers

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

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